I am a huge fan of the DoT and run technique. Demo, or Destro for that matter, is much faster on single target mobs them Affliction. With Affliction though you should be able to easily kill 10 mobs at your level at a time and do that 4 or more times without resting. Keep the Succy out, turn off anything that uses mana (lash of pain etc) her mana is yours with Dark Pact. DoT crap up and run around. Siphon life x10 will keep your HP up, if you need to drain tank one after it is all dotted up.

I haven't seriously played a lock in a while but I did this leveling solo and was doinf 4-6 mobs at a time. The only thing that has ever leveled faster for me was my prot pally AoE hunting.